1. Field of the Invention
This invention relates to a magnetic encoder for performing positional detection by converting a change in magnetic field into an electric signal with respect to elements having a so-called magnetic resistant effect in which an electric resistance varies in accordance with an incident magnetic field, and more particularly, the invention relates to an improvement of the magnetic encoder in which the signal-detecting accuracy is enhanced.
2. Description of the Related Art
A highly accurate positional sensor or rotary sensor is necessary in a wide variety of technical fields, including the fields of NC machine tools, robots, OA instruments, VTRs, etc. In these industrial fields, various servo-motors and rotary encoders are utilized. With the development of OA and FA technologies, sensors having higher operating speeds and reliability are desired. Under these circumstances, for instance, conventional optical techniques are often used to realize a rotary sensor. However, such a sensor is composed of semiconductor elements such as photo-cells and LEDs, and is thus sensitive to dust and insufficiently stable in response to a change in temperature. Also, optical sensors suffer the additional drawbacks of numerous composing parts and complicated structures.
Accordingly, in recent years, magnetic type sensors using the magnetic resistance effect have been developed. Such sensors have a higher detecting accuracy and temperature stability, and are relatively insensitive to dust, and thus they are applied in many fields.
FIG. 5 shows a schematic oblique sectional view of the conventional magnetic rotary sensor. A rotary drum 51 rotates together with a rotary shaft 52, for instance, in the rotational direction identified by the arrow. A magnetic recording medium layer is provided on the peripheral surface of the rotary drum 51 in the form of NS-magnetic poles. The magnetic recording medium layer, for instance, may be magnetic coating films such as gamma-iron, etc., or may be formed of strontium ferrite, etc.
The detecting accuracy conforms to the number of magnetic poles selected. When there are several hundred to several thousand magnetic poles, the width of a single magnetic pole will be formed by a very narrow micro-magnet.
A magnetic sensor 53 is arranged to confront the magnetized surface of the rotary drum 51 and is provided with a magnetic resistant element unit 54 formed of plural magnetic sensing patterns (consisting of strongly magnetized thin film patterns having the magnetic resistant affect) formed on a glass board. The magnetic sensor 53 is spaced a specified interval G within the magnetizing range of the magnetic field of the micro-magnets of the rotary drum 51.
Upon rotation of the rotary drum 51, the magnetic field on the surface of drum 51 travels towards the confronting magnetic resistant element unit 54, whereby each magneto-sensitive pattern of the magnetic resistant element unit 54 undergoes electro-magnetic conversion in which the strength of the incident magnetic field changes the resistance value of the pattern. Thus, an electric signal is issued in response, and detection is effected of the rotary speed and position of the drum 51.
The conventional magnetic encoder as described above and as shown in FIG. 5 includes a magnetic drum for repeatedly generating a magnetic signal and a magnetic sensor confronting the magnetic drum in non-contact parallel form through the specified interval G. Also, the magnetic resistant element unit of the magnetic sensor is composed of plural magneto-resistive patterns coupled to electric components. According to such a magnetic encoder, for instance, it is possible to promptly and accurately detect the position of a moving object and/or the rotary speed of motors such as numerical control (NC)-machines or robots.
As the above mentioned control systems are desired to achieve various functions and higher accuracy, compact and high performance detecting instruments are desired as well. For instance, a magnetic encoder having high and low rotary speed and/or positional detection with high accuracy is desirable. To meet such requirements, it is necessary to generate an output signal or electric signal having different pulse numbers. However, the conventional magnetic encoder, is as mentioned before, arranged having one magnetic pole line on the peripheral surface of a rotary drum, and thus cannot realize such requirements. Further, the provision in combination of plural magnetic encoders would result in a complicated structure having numerous parts, and thus poses problems with respect to the desirability of a compact device.
Accordingly, the present inventors have solved these problems as shown in FIG. 10 by the provision of an increment phases I.sub.1, I.sub.2, I.sub.3 composed of plural magnetic pole lines having different magnetizing pitches (different numbers of magnetic pole arrangements) and a Z-phase for inducing a standard point signal on the peripheral surface of a rotary drum 91. Also, the magnetic encoder is further provided with a sensor 93 composed of independent magnetic resistant effect element units 94a, 94b, 94c respectively confronting each increment phase I.sub.1, I.sub.2, I.sub.3. By such a composition, output signals having different pulse numbers can be induced. Thus, for instance, a low-pulse number is applied for high-speed rotation, and a high-pulse number is applied for low-speed rotation, and thereby a highly accurate detection can be made. Simultaneous detection can be made for the signals of the low-pulse number and the high-pulse number, whereby subsequent circuit-processing can make further synthesis of a more accurate signal pulse.
However, as a result of various review tests, when the increment phases I.sub.1, I.sub.2, I.sub.3 having different magnetic pitches are arranged on the same rotary drum as described above, the following problems have been found. As shown in schematic views FIG. 11 (a) and (b), the provision of numerous magnetic poles having a shorter distance between poles gradually results in a shorter travelling distance of the effective magnetic flux .PHI. even when the magnetic characteristics of the magnetic recording medium itself would be the same. Thus, the value of the output signal caused at the magnetic resistant element appears differently. For instance, the signal waveform detected by magnetic resistant elements a.sub.1 and b.sub.1 located at the same distance x from the medium surface will appear as shown in FIG. 11 (a') and (b'), and the level of signal, and waveform thereof, and the waveform-distortion are different. Thus, it is difficult to practically apply such an arrangement, and no realization of such a magnetic encoder for precision instruments was found.
This invention is intended to offer a magnetic encoder for outputting plural detecting signals having an equal output value and waveform-shape.